1. Field of the Invention
This invention relates to a vibration isolator for a motor which is used for the purpose of preventing such stepping motors as are serving to feed papers in a facsimile or a copying machine or operate a printing head in a printer from generating vibration or emitting noise.
2. Description of the Related Art
In recent years, vibrations and noises emanating from various office automation (OA) machines copiously adopted for clerical works in consequence of the automation of business offices have come to pose a problem. One of the causes for the problem resides in the fact that when the motors built in the OA machines are in operation, the vibrations of these motors are transmitted to the frames of the machines. The practice of interposing a vibration isolator between the motors and the frames of the machines has been heretofore resorted to for precluding transmission of vibration and abating noise. FIG. 1A and FIG. 1B illustrate a conventional vibration isolator 1 to be used in the installation of a stepping motor. This vibration isolator 1 has a rubber body 4 (made of chloroprene or butyl rubber, for example) attached by means of vulcanization between two metal pieces containing fixing holes 2, . . . and is provided at the center thereof with a through hole 5 for allowing insertion therethrough of the drive shaft of a stepping motor (JP-U-1-71,964). This vibration isolator 1 is used as in a platen drive mechanism for a printer illustrated in FIG. 2 or in a paper feeding mechanism illustrated in FIG. 3, for example. It is inserted between a stepping motor 6 and a frame 7 and fastened thereto with bolts which are fitted into the fixing holes 2, . . . .
In the conventional vibration isolator 1, however, the spring constant thereof in the axial direction (namely in the direction of compression) is 1, whereas the spring constant thereof in the direction perpendicular to the axis (namely in the direction of shearing) is 0.2. Because of the small spring constant in the shearing direction as compared with that in the axial direction, this vibration isolator 1 is at a disadvantage in tending to be affected by a displacement in the shearing direction and being deficient in accuracy of positioning. In the case of such a gear drive device as the platen drive mechanism of the printer of FIG. 2, when a pinion 9 disposed around a drive shaft 8 of the stepping motor 6 rotates a gear 12 disposed round a platen shaft 11 of a platen 10, the drive shaft 8 and the pinion 9 are inevitably displaced in the shearing direction in response to the vibration of the stepping motor 6 because the spring constant of the vibration isolator 1 in the shearing direction (in the direction perpendicular to the drive shaft 8) is small. As a result, the backlash between the pinion 9 and the gear 12 is fluctuated to degrade the accuracy of positioning. In the case of such a belt drive device as the paper feeding mechanism of FIG. 3, since a pulley 13 disposed round the drive shaft 8 of the stepping motor 6 is connected to another pulley 14 through the medium of a belt 15, the tension of the belt 15 acts on the pulley 13 and the drive shaft 8 and consequently displaces the drive shaft 8, for example, in the shearing direction (in the direction perpendicular to the drive shaft 8).
For the elimination of these drawbacks, measures such as increasing the hardness of the rubber body 4 and decreasing the thickness thereof which are intended to increase the spring constant of the vibration isolator 1 in the direction perpendicular to the shaft thereof have been adopted. The effect of isolating vibration, however, is rather degraded when the spring constant in the direction perpendicular to the shaft is increased by these measures.
Incidentally, rubber has a thermal insulation. When the stepping motor 6 is set to driving as fixed on the frame 7 through the medium of the vibration isolator 1, therefore, the heat generated in the stepping motor 6 is insulated by the vibration isolator 1 and accumulated in the stepping motor 6 itself. The stepping motor 6, therefore, is at a disadvantage in acquiring satisfactory properties only with difficulty and suffering a decrease in service life.